The present invention relates to a humidifier, a fuel cell system utilizing the same and a humidification process, and particularly to a humidifier utilizing a hollow fiber membrane, a fuel cell system utilizing the same and a humidification process.
Fuel cell systems, especially solid polymer fuel cell systems are widely known as a power source for electric vehicles. In such fuel cell systems, a humidifier is used to moisture-exchange off gas, viz. moist gas discharged from a fuel cell, between its moisture and air as a dry gas, and to generate humidified air or humidified gas. Preferably, a humidifier used with such fuel cell systems is of a lower power consumption type, and is required compactness with less attachment space. For this reason, among many other humidifiers, such as a supersonic humidifier, a steam humidifier, a vaporizing humidifier and a nozzle injection type humidifier, a humidifier utilizing water permeable membranes, especially hollow fiber membranes is commonly used with a fuel cell.
A conventional humidifier utilizing hollow fiber membranes is disclosed in Japanese Laid-open Patent Publication No. HEI-7-71795. As shown in FIG. 14, a humidifier 300 comprises a housing 300, on which is provided a first inlet 302 for introducing dry air and a first outlet 303 for discharging the dry air (humidified dry air). A bundle of hollow fiber membranes 304 consisting of a number of for example 5000 hollow fiber membranes is accommodated within the housing 301.
At both ends of the housing 301, fastening members 305, 305xe2x80x2 are provided for fixing the ends of the bundle 304 while leaving them open. Outside of the fastening member 305 is provided a second inlet 306 for introducing moist air or moist gas, and a second outlet 307 is provided outside of the fastening member 305xe2x80x2 for discharging the moist air, moisture of which is separated and removed at the bundle of hollow fiber membranes 304. The fastening members 305, 305xe2x80x2 are covered with a first head cover 308 and a second head cover 309, respectively. And the second inlet 306 is formed on the first head cover 308, while the second outlet 307 is formed on the second head cover 309.
In the aforementioned humidifier 300 utilizing hollow fiber membranes, the moist air introduced from the second inlet 306 passes through the hollow fiber membranes forming the bundle of hollow fiber membranes 304, and the moisture within the moist air is separated by capillary action of the hollow fiber membranes. The separated moisture moves outward of the hollow fiber membrane through a capillary tube of the membrane. The moisture-removed air is discharged from the second outlet 307.
Meanwhile, dry air is supplied from the first inlet 302. The dry air from the first inlet 302 flows outside of the hollow fiber membranes forming the bundle of hollow fiber membranes. Because the moisture separated from the moist air has moved outside of the hollow fiber membranes, the moisture humidifies the dry air. The humidified dry air is then discharged from the first outlet 303.
However, in the conventional humidifier 300 shown in FIG. 14, the first air inlet 302 which introduces the dry air is formed on the housing 301 at the side near the center of the lengthwise direction thereof. For this reason, as shown in the black arrow of FIG. 14, the dry air flowing outside of the hollow fiber membranes in the bundle 304 of the hollow fiber membrane stored within the housing 301 flows the central portion in the lengthwise direction within the housing 301. Consequently, the areas S residing near the ends of the bundle 304 of the hollow fiber membranes do not contribute to the exchange of water in a sufficient manner and, thus there is a problem that the ratio of moisture recovery at such portions unduly low in relative to the permeable water within the hollow fiber membranes.
The object of the present invention is to make it possible to water exchange in a sufficient manner even at the portions near the ends of the hollow fiber membranes of the bundle of the hollow fiber membranes stored within the housing to thereby improve the ratio of moisture recovery of the humidifier.
According to the present invention which can attain these and other objects, there is provided a humidifier having a plurality of water-permeable hollow fiber membranes placed along the lengthwise direction of a housing accommodated within the housing in which gases each having a different moisture content flow inside and outside said hollow fiber membranes to carry out moisture exchange whereby the dry air having a low moisture content is humidified, said humidifier comprising: a bypass channel, in which the gas flowing outside the hollow fiber membrane, formed on an approximately central portion of the cross-lengthwise direction of said housing along the lengthwise direction of said housing, said bypass channel having a diameter larger than that of said hollow fiber membrane, and an inlet which introduces the gas flowing outside the hollow fiber membrane into the housing and an outlet which discharges the gas flowing outside the hollow fiber membrane formed on said bypass channel.
The humidifier of the present invention has a bypass channel formed on an approximately central portion of the cross-lengthwise direction of said housing along the lengthwise direction of said housing. Since the bypass channel has a diameter larger than that of said hollow fiber membrane, the gas flowing outside the hollow fiber membrane is introduced into the bypass channel. By being passed through the bypass channel, the gas flowing outside the hollow fiber membrane can be transferred to every areas of the housing. Consequently, since the gas can be supplied over the entire area of the bundle of the hollow fiber membranes, the moisture exchange can be effectively carried out even at an end of the housing, enhancing the moisture recovery.
The term xe2x80x9can approximately central portion of the cross-lengthwise direction of the housingxe2x80x9d used herein does not have a meaning of being completely accorded with the center of the cross-lengthwise direction of the housing (the center of the axis), but means to encompass the portion slightly out of the center. Furthermore, it is possible for the formation of the bypass channel if the circumference of the bypass channel is surrounded by the hollow fiber membranes.
In a preferred embodiment of the humidifier according to the present invention, a plurality of the outlets which discharge the gas flowing outside the hollow fiber membrane are formed on said bypass channel at several distance.
According to this embodiment, a plurality of the outlets which discharge the gas flowing outside the hollow fiber membrane are formed on said bypass channel at several distance.
In this embodiment, since a plurality of the outlets which discharge the gas flowing outside the hollow fiber membrane are formed on said bypass channel at several distance, the gas flowing outside the hollow fiber membrane can be discharged in the lengthwise direction in a substantial uniform manner. For this reason, the gas flowing outside the hollow fiber membrane can be supplied to substantially the entire areas of the housing. Accordingly, the moisture exchange can be carried out with higher efficiency.
According to this embodiment, an inlet port for the introduction of the gas flowing outside the hollow fiber membrane into the housing and an outlet port which discharges the gas flowing outside the hollow fiber membrane are formed on the housing, and said inlet port and said outlet port are placed opposite each other beyond the bypass channel.
In this embodiment, an inlet port for the introduction of the gas flowing outside the hollow fiber membrane into the housing and an outlet port which discharges the gas flowing outside the hollow fiber membrane are formed opposite each other beyond the bypass channel. For this reason, the gas flowing outside the hollow fiber membrane introduced from the inlet port flows outside the hollow fiber membrane, while moving in the cross-lengthwise direction of the housing. Consequently, the distance of the movement of the gas flowing outside the hollow fiber membrane becomes longer and, thus, the moisture exchange can be carried out in a sufficient manner, making it possible to enhance the moisture recovery.
The humidifier according to the present invention having such a construction can be advantageously utilized as a humidifier for the fuel cell system.
According to the present invention, there is also provided a humidification process utilizing a hollow fiber membrane module comprising a plurality of water-permeable hollow fiber membranes placed along the lengthwise direction of a housing accommodated within the housing, in which gases each having a different moisture content flow inside and outside said hollow fiber membranes to carry out moisture exchange whereby the dry air having a low moisture content is humidified, comprising: a step for subjecting one of said gas to flow in the bypass channel; a step for subjecting said gas from the bypass channel to flow outside the hollow fiber membrane; and a step for carrying out a moisture exchange between said gas flowing outside the hollow fiber membrane and the gas flowing inside the hollow fiber membrane.